This invention relates, generally, to liquid natural gas (LNG) delivery systems and, more specifically, to a high pressure LNG delivery system particularly suited for use on natural gas powered motor vehicles.
In order to avoid dependence on foreign sources of fuel oil, great efforts have been made to find a cheap and reliable domestic energy alternative. One such alternative is natural gas (NG) which is domestically available, plentiful and relatively inexpensive and environmentally safe as compared to oil. Because one of the largest uses for oil is as a fuel for motor vehicles, great efforts have been made to develop natural gas powered engines.
Some proposed engines require that the intake pressure of the NG be at elevated pressures, i.e. 300 psig or the like. This presents a particular problem when one wishes to utilize LNG as the vehicle fuel because LNG is preferably stored at the range of 15 to 50 psig where it is very dense.
One such engine is a dual-fuel modified diesel engine which runs on a 60/40 LNG to diesel fuel mixture. While this engine substantially reduces diesel fuel consumption, it requires that LNG be delivered to the engine at approximately 300 psi, a pressure approximately 6 times the normal storage pressure for LNG. This extremely high pressure causes storage and handling problems for the volatile LNG. These problems are magnified by the fact that when the LNG is carried on a motor vehicle, it is exposed to relatively high temperatures and constant motion. Of particular concern is the difficulty in pressurizing the LNG because the constant motion of the vehicle causes the LNG to mix with the natural gas vapor pressure head thereby condensing the natural gas vapor and collapsing the pressure head. This causes all the stored LNG to heat up to an equilibrium temperature--near that of 300 psig--whereby it increases in volume to a point where it could "liquid over fill" the tank.
To compensate, the tank capacity at time of fill cannot be fully utilized, thus undesirably limiting the range of the vehicle. Also for a tank to hold 300 psig it must have a reserve pressure (to accept pressure rise when fueled, but not in use) and a 500 psig rating would be considered normal. Pressure tanks which safely contain 500 psig require much thicker and heavier walls than those which contain 50 psig, and this additional weight reduces the net payload of the vehicle, also an undesirable condition.
Another proposed method of providing 300 psig intake pressure from LNG stored at 15 psig is to provide a pump, whose intake pressure is storage pressure (15-50 psig) and discharge pressure is 300 psig or the like. However, pumps that dependably supply liquid at a rate proportionate to their speed--a desirable function when supplying fuel to an engine where fuel supply determines the vehicle speed--require a Net Positive Suction Head (NPSH). At standard cryogenic pump installations, various methods are utilized to obtain NPSH, but most involve stratification and/or hydrostatic head (i.e. sub-cooling) in the pump supply tank. However, tanks containing cryogens (i.e. LNG) tend to quickly destratify and come to equilibrium throughout when vibrated, as would normally occur when a bus or truck is in motion. Such being the case, a vehicle pump can experience varying NPSH (in fact, as low as 0), thus varying volumetric efficiencies--ranging from no flow to high flow. To a vehicle operator this would produce difficult to control engine/vehicle speed variations, a potentially unsafe condition.
Adding a post-pump reservoir and substitute regulator control to smooth out these variations has also been suggested. However, such a reservoir represents high pressure compressed natural gas ("CNG") and constitutes considerable additional equipment. In addition, such a system has difficulty dealing with the boil-off gaseous NG from its stored LNG.